1. Field of the Invention
This invention relates to an alkali metal dispenser useful in electron tubes for vapor deposition of an alkali material onto a photoemissive surface within the tube.
2. Description of the Prior Art
The deposition of alkali metal vapors for the sensitization of photocathodes in photomultiplier tubes, camera tubes and other electron tubes having photoemissive surfaces is often accomplished by alkali dispensers or evaporators mounted within the tube near the photocathode surface. The known alkali dispensers typically comprises a metal holder which contains a mixture of a chromate or bichromate of a relevant alkali metal and a reducing agent. The holder is generally made of a relatively thin metal sheet that is rolled into a substantially cylindrical container having a slight overlap as shown, for example, in U.S. Pat. No. 3,945,949, issued on Mar. 23, 1976. The overlapping portions of such a container is often spot welded at spaced intervals along the overlap as described in U.S. Pat. No. 3,372,967, issued on Mar. 12, 1968 and U.S. Pat. No. 3,644,101, issued on Feb. 22, 1972. A wire lead is generally attached to each end of the container to pass current through the container for heating the alkali compound. The resulting alkali vapor escapes through the seam between the weld spots into the cathode area.
One of the major problems encountered with these known alkali dispensers is the uncontrollable escape of alkali metal particles. Large number of welds typically provides poor integrity of the alkali metal container often resulting in open welds that undesirably allow escape of alkali metal particles. Also, due to variation in welding, repeatedly depositing consistent amounts of alkali metal is extremely difficult to achieve. In addition, tantalum, the most attractive housing material, is difficult to spot-weld, often resulting in marginal welds and hence reduced reliability. Lastly, the welding operation is skill dependent resulting in a relatively costly package.